In a typical high dynamic range display, a backlight produces a comparatively low resolution image on an inward side of a light modulator. The low resolution image is modulated by the light modulator to provide a comparatively high resolution image, which appears on the outward side of the light modulator for perception by an observer. The modulator may comprise an LCD. Electronic signals for controlling the backlight and the light modulator may be generated by suitable control circuits of known types. For example, an LCD light modulator may be controlled using control circuits of the same type used to control the LCD modulator in a conventional LCD computer monitor. Example high dynamic range displays are disclosed in international patent publication WO 02/069030 published 6 Sep. 2002 and in international patent publication WO 03/077013 published 18 Sep. 2003, both of which are hereby incorporated by reference herein.
Multiple light sources within the backlight may illuminate each pixel of the light modulator. Maintenance of a relatively small separation distance between the backlight and the modulator allows adjacent pixels of the low resolution image to merge smoothly into one another. Well known image compensation techniques may be applied to remove any undesirable image-blurring artifacts.
One difficulty with such image compensation techniques is that the light intensity (luminance) distributed from a pixel of the low resolution image to a corresponding pixel of the high resolution image is ideally invariant. Otherwise, the intensity of the high resolution image's pixels may vary as a function of the direction from which the image is viewed by an observer, which is undesirable. One common method of achieving such luminance invariance is to incorporate in the display a diffuser having a Lambertian output distribution (i.e. the angular distribution of light rays emanating from the outward side of the diffuser is symmetrical about the diffuser's normal direction and is independent of the direction of the corresponding incident light rays). Such diffusers eliminate parallax (apparent changes in the direction of an object, due to changes in the observer's position which correspond to different lines of sight to the object). Accordingly, the observer does not see unwanted images of things behind the display layer.
Lambertian diffusers undesirably reduce the display's overall luminance by a factor of about ten. This is partly due to backscatter of light rays within the diffuser, and partly due to the diffuser's inherent function of spreading luminous flux over a relatively large solid angle (effectively π steradians, compared to a preferred solid angle of no more than about 0.5 steradians).
The inventors have recognized a need for display apparatus that preserves image luminance in display backlights by confining incident light rays within a preferred angular viewing range, in a manner which reduces viewing parallax by restricting the dependence of the angular distribution of light rays transmitted through the display on the direction from which an observer views the display.